The present invention relates to a planetary gear type reduction gear with torque limiter, and more particularly to a planetary gear type reduction gear with torque limiter that moves a friction member in its axial direction relative to an inner gear to utilize its friction resistance.
A reduction gear mechanism of a type that outputs with a base of large reduction gear ratio by incorporating a planetary gear type reduction gear on a shaft of a DC motor has been known to the art.
Because of a construction of the DC motor that rotates a shaft by applying electric current to its coil, there is a characteristic that it is weak against a large load from the side of a shaft. Namely, in case a certain load is exerted to the shaft and a condition of the rotation of the motor shaft being stopped continues, the electric current applying to the coil becomes high, and as a result, the inside of the motor turns to the high temperature which results in seizure, melting an insulating material, causing a circuit short to cause a faulty contact which are some of the problems.
Furthermore, in case where a reduction gear mechanism with large reduction gear ratio is provided, rpm of an output shaft of the reduction gear mechanism becomes greatly smaller relative to rpm of the motor shaft so that the rotary torque becomes extremely great inversely at the output shaft of the reduction gear. In this condition, in case where large load is exerted on the output shaft of the reduction gear mechanism, which results in the great load on the gear portion in the reduction gear mechanism so that this undue large load causes a breakdown.
In order to solve the foregoing problems, in an assembly in which the planetary gear type reduction gear is incorporated in the shaft of the DC motor, a torque limiter is interposed in the assembly to form a structure of not transmitting the rotation in case a load of predetermined level is exerted on the output shaft of the reduction gear, the rotation is not transmitted, which had been conceived (for example, refer to official gazette of Japanese Patent laid open publication, KOKAI No. 2003-13014 6). This patent literature disclosed a construction wherein an O-ring that is a friction member is interposed between an outer periphery of an inner gear and an inner periphery of a case that is a stationary member is disclosed.
The planetary gear type reduction gear is constructed generally in that a central gear is provided on a rotary shaft of a motor corresponding to an input shaft, and a planetary gear is meshed with the central gear, and an inner gear is caused to mesh with the planetary gear and a carrier is provided to axially support the planetary gear. In case, a torque limiter wherein an O-ring that is a friction member is interposed in a radial direction between the outer periphery of the inner gear and the inner periphery of the case that is a stationary member is provided on this planetary gear type reduction gear, it is necessary that an axis of the inner periphery of the case that is the stationary member and an axis of the outer periphery of the inner gear must completely coincide with each other.
If the coincidence is not obtained, namely, the outer periphery of the inner gear is rotatively displaced relative to the inner periphery of the case, the contact pressure of the O-ring is resulted in the change, and also, the pressure contact in the bearing portion of the inner gear becomes large which renders the influence leading to the instability of the transmission torque at the rotative displacement of the inner gear. However, it is an extremely difficult thing to cause a complete coincidence of the axis of the inner periphery of the case that is the stationary member and the axis of the outer periphery of the inner gear, and as a result, such a structure will become an extremely expensive from the standpoints of the construction cost and of the precision level.
Moreover, with respect to those inventions of prior art disclosed in the patent literatures, both end portions of the stationary inner gear and the movable inner gear are brought to a contact condition with the inner end surface of the case so that it is subjected to getting the resisting power by the friction contact at this portion, and which forms a factor of causing the transmitting torque to the instability.
On the other hand, where a plurality of stages of the planetary gear type reduction gear mechanism are provided, and if a torque limiter is incorporated into the mechanism as a generally conceived structure, a reduction gear mechanism is connected to the DC motor, and a construction is conceived in that a torque limiter is caused to fit to the output shaft of the final stage reduction gear mechanism of the reduction gear mechanism.
However, this method has the following drawbacks. Namely, in case the rpm of the motor shaft is reduced, the bigger the reduction gear ratio, the rotary torque as a matter of fact becomes bigger. In other words, the later stage of the reduction gear mechanism, the more the reduction gear ratio becomes bigger, the rotary torque becomes large. In case the torque limiter is incorporated onto the output shaft of the final stage reduction gear mechanism of the reduction gear mechanism, the rotation is transmitted or cut at the portion where the rotary torque is the largest. In the construction where the transmission or cutting of the torque is designed to take by the friction force of the friction member, where the rotary torque is large, the large friction force is required for the torque transmission, and as a result, it cannot be avoided that the torque limiter mechanism becomes a large size.
Furthermore, in case of incorporating the torque limiter to the output shaft of the final stage reduction gear mechanism of the reduction gear mechanism, the reduction gear ratio at the final stage fluctuates depending on the design so that the rotary torque becomes different, and along with the fluctuation, torque limiter mechanism of different designs must be prepared. In other words, a common design to all the torque limiter mechanisms cannot be made.
A primary object of the present invention is to provide a planetary gear type reduction gear with torque limiter at a low cost that demonstrates always a torque limiter function in stable mode which keeps a constant contact pressure of a friction member such as an O-ring or the like in case the outer periphery of the inner gear is rotatably displaced relative to the inner periphery of the case even though the axis of the inner periphery of the case that is the stationary member and the axis of the outer periphery of the inner gear are not completely coincided, and also the contact pressure at the bearing portion of the inner gear does not influence on the transmission torque.
A secondary object of the present invention is to provide a planetary gear type reduction gear with torque limiter that does not lower the transmitting torque even though the friction member such as O-ring and the like wears off due to the use of a long period of time.
A tertiary object of the present invention is to provide a torque limiter mechanism in a small size in case of providing a plurality of stages of the planetary gear type reduction gear mechanism.
A fourth object of the present invention is to provide a common design of the torque limiter mechanism.
The fifth object of the present invention is to manufacture a limiter torque of accurate type matched with requirement without preparing many kinds of friction member such as O-rings of different cross sectional diameters or without preparing many kinds of pressing means of different pressing powers.
Furthermore, an object of the present invention is to facilitate simple torque adjustment of the limiter torque which matches using conditions of the customers.